The invention relates to a pressure transducer for determining the pressure in the combustion chamber of an internal combustion engine and comprising a housing and a piezo-electric crystal located in the housing between a plunger and an abutment. In such a known pressure transducer, the pressure curve with respect to time in the combustion chamber of an internal combustion engine is determined via the affect of force on one or more quartz crystals utilizing their piezoelectric properties. The elements transmitting the force are pressed against the quartz crystals with high mechanical prestress. However, these pressure transducers have large dynamic temperature errors and become soiled relatively quickly. In addition, there is a higher signal noise as a result of the purely mechanical force-locking connection. The known pressure transducers are not sufficiently independent of temperature fluctuations. Dynamic temperature errors are caused by additional forces acting on the quartz crystals in the engine cycle as a result of the periodic temperature changes in the area near the surface of the combustion chamber.
A portion of the force acting on the front face of the pressure transducer face is formed by a diaphragm and the rim of the housing, is absorbed directly by the housing, the remaining portion is likewise absorbed by the housing indirectly via the plunger, quartz crystals and the counter-bearing or abutment. In so doing, the force K acting on the quartz crystals is based on the following equation: ##EQU1##
K=force
p=pressure
A.sub.eff =effective surface
c.sub.M =spring constant of the diaphragm
c.sub.S,Q =spring constant of the plunger with quartz crystals
The force K acting on the quartz crystals is accordingly substantially determined by the effective surface A.sub.eff of the pressure/force conversion. The diaphragm is conceived as a spring which is prestressed at the housing rim and is elastically deformed when loaded due to the contraction of the plunger. Accordingly, a part of the force component determined by p. A.sub.eff is again directly absorbed by the housing due to the force by-pass through the diaphragm ring. This component is a function of the ratio of the spring constants of the diaphragm C.sub.M and the plunger with the quartz crystals c.sub.S,Q. For an unequivocal relationship between the measured force K and the pressure p, the effective surface A.sub.eff and the spring constants c.sub.M and c.sub.S,Q must be sufficiently independent from the pressure and temperature. These dependencies do not have a disturbing effect when the spring constant c.sub.M is sufficiently small.
However, the allowable maximum value for the diaphragm spring constant c.sub.M is still smaller because of the requirement for the suppression of dynamic temperature errors. For this, the spring constant c.sub.M must be as small as possible, thus the diaphragm must be as thin as possible. This requirement is not met in the known pressure transducers.